The compound 1-[2-(2,4-dimethylphenylsulfanyl)phenyl]piperazine (compound I) has been disclosed in WO 03/029232 as the free base. Compound I has the molecular structure depicted below.

Different pharmaceutically acceptable acid addition salts of Compound I has been disclosed in WO 2007/144005, including the lactic acid addition salt.
Compound I has been reported to exert serotonin transporter inhibition (WO 03/029232) and is said to be useful for the treatment of affective disorders, e.g. depression and anxiety. In addition, compound I exerts 5-HT3 antagonism and 5-HT1A agonism which suggests this compound to be useful e.g. in the treatment of cognitive impairment in depressed patients, and the treatment of pain and residual symptoms in depression (WO 2007/144005 and WO 2008/113359).
In vitro and in vivo experiments conducted with compound I describing receptor efficacy and disease pharmacology have been outlined in WO 03/029232, WO 2007/144005 and WO 2008/113359.
Compound I has been tested in clinical trials in patients using HAM-D (Hamilton Rating Scale for Depression) as clinical end-point; for details see WO 2008/113359. The HAM-D scale may be used to assess the severity of depression in patients by means of a 24 items questionnaire. According to the outcome of the clinical study compound I is believed to be particularly useful in the treatment of depression avoiding sleep and sexual related adverse events (WO 2008/113359).
For many pharmaceutical compounds, oral administration of a tablet, capsule, pill or similar intended for swallowing is the preferred administration form. However, some patients, e.g. elderly and paediatric patients may have difficulties swallowing, and liquid solutions may be a suitable alternative avoiding the need for swallowing tablets, capsules, pills, etc. A liquid solution further provides a possibility of a flexible dosing regime. In order to limit the volume of a liquid solution it is necessary to have a high concentration of the active ingredient in the solution, which again requires a high solubility of the active ingredient.
The present invention is related to liquid formulations of compound I.